The production of magnesium metal by electrolysis of magnesium chloride concentrate, as presently practiced commercially, is generally carried out as a batch-type operation in a series of electrolytic cells in each of which the electrolyte comprises a substantially anhydrous magnesium chloride concentrate derived from the naturally occurring brines of salt lakes, inland seas, oceans and the like. However, before these naturally occurring brines can be used as an electrolyte for the electrolytic recovery of magnesium metal they are subjected to solar evaporation in successive ponds to precipitate a major portion of the NaCl and a complex salt of KCl and MgSO.sub.4 known as kainite as described in U.S. Pat. No. 3516785. The residual brine, hereinafter referred to as a holding pond brine is then desulfated by treatment with calcium chloride and the desulfated brine concentrated to effect removal of residual metal salts and water. In this connection it is well understood that a magnesium chloride electrolyte derived from brine must be substantially free of water of crystallization for otherwise H.sub.2 will be formed at the cathode; and O.sub.2 and compounds of oxygen will be evolved at the anode thus causing rapid deterioration of the anodes. Moreover, any MgOHCl and MgO must be removed otherwise particulate MgO will form and sink to the bottom of the cell to form cell smut which reduces cell efficiencies and requires much labor and expense, in terms of cell down time, to remove it from the cell.
While it is possible to remove up to four moles of water from the magnesium chloride brine by slow drying the brine at temperatures of about 400.degree.F further drying at elevated temperatures has been found to effect hydrolysis of the brine with the result that a mixture of magnesium chloride and magnesium hydroxychloride is produced. The present practice of forming a substantially dry magnesium chloride feed material for electrolytic cells is by using a spray-drier unit. Units of this type are traversed by heated gases having air inlet temperature of about 1000.degree.F and an outlet or discharge temperature of about 500.degree.F. A magnesium chloride brine concentrate fed into the inlet of the spray drier is recovered from the outlet as a substantially dry product containing no more than from 3 to 5% water--which, after further dehydration in a melt cell, is sufficiently free of water to be used effectively as feed material in electrolytic cells for the product of magnesium metal. However while substantially free of water the spray dried magnesium chloride prepared as described above will be in the form of thin walled microspheres about 800 microns in size and easily friable into fragments which, because of their low bulk density are easily air borne and hence create difficult problems of handling, transportation, storage and the like. Also, it is well known that spray dried magnesium chloride prepared from brine concentrate will contain boron or a boron compound which if present in the magnesium chloride electrolyte in proportions equivalent to as little as from 150 to 200 ppm boron will inhibit coalescence of the magnesium metal in the cell with the result that cell current efficiencies are lowered and significant amounts of magnesium metal end up in the cell smut.
It is important therefore to production of magnesium metal on plant scale that the magnesium chloride electrolyte be not only substantially free of MgO and/or MgOHCl but also boron; or that the level of boron in the electrolyte be reduced sufficiently that its adverse affects on the coalescence of the magnesium metal and cell efficiencies be minimized.
Known methods for producing magnesium chloride electrolytes having low levels of boron and other contaminants including MgO and/or MgOHCl include chlorination of the spray dried magnesium chloride i.e. treating the MgCl.sub.2 with gaseous chlorine in a reducing atmosphere, either during melting of the spray dried material or subsequent thereto. However, while chlorination of the MgO and/or MgOHCl is readily effected the removal of boron by chlorination is a slow and expensive process requiring long contact times in the melt cells and/or chlorinators, and the handling of large volumes of gaseous chlorine. As a result chlorine cost become prohibitively high, melt cell life is seriously shortened and catastrophic feeding and corrosion problems develop. And further, because of the fluffy, friable nature of the spray dried magnesium chloride it is extremely difficult to feed it to the melt cell and/or chlorinators at controlled rates; and is frequent cause for plugging and other operational difficulties.
To recapitulate, using present methods of forming a magnesium chloride electrolyte from naturally occurring brines for the electrolytic production of magnesium metal the presence of boron in the electrolyte and the light weight and easily friable nature of the spray dried magnesium chloride concentrate are the source of many difficult handling problems, require elaborate and expensive equipment installations, and give rise to high operational and maintenance costs, low cell efficiencies and low recoveries of magnesium.
It is desirable therefore to provide an improved method for treating naturally occurring brines so as to form a substantially anhydrous high density, boron-free magnesium chloride feed material for the electrolytic production magnesium metal in a simple, relatively economical and efficient manner.